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The Cold, Hard Facts about Winter Storms

By Steven Hilberg and adapted by Jim Angel, state climatologist

Introduction

As the sun gradually lowers in the sky in the Northern
Hemisphere during the fall, cold arctic and polar air masses intrude farther and farther
south into the United States. Disturbances forming along the boundary between the cold
polar air and the relatively warm, tropical air sometimes turn into winter storms. These
are usually large, intense low-pressure systems that may cover tens of thousands of miles.
Illinois' location in the Midwest and its great north-to-south extent place it in the path
of many of these storms. When conditions are right, these storms can strike Illinois hard,
leaving snow and ice over all or parts of the state.

Severe winter storms in Illinois produce more total
damage than any other form of short-term severe weather, including tornadoes, lightning,
and hail. Central Illinois has the distinction of being in the nation's primary area for
severe freezing rain (ice) storms. However, any part of the state is apt to have a severe
snow storm or ice storm.

What is a severe storm? How does it impact our daily
lives? How can we prepare for it? What should you do and not do when a winter storm
strikes?

How Many Storms and When?

Illinois, on the average, experiences five severe
winter storms during the November-April period. These storms may be those with only
heavy snow, or with snow and ice mixed, or with ice (glaze) only. Although the average is
five per winter, as many as 18 storms have occurred in one winter (1977-1978) and as few as two
(1921-1922). The number of times severe winter storms have occurred in the various
sections of Illinois is shown in Figure 1.

January is the most favored month for severe winter
storms, although December, February, and March are close behind in numbers of storms. The
earliest a severe winter storm ever hit Illinois was on October 28-30, 1925, and the
latest one ever to hit was on May 1-2, 1929. The maps in Figure 2
depict the snowfall patterns resulting from these storms.

A study of the number of times that severe winter storms
have occurred on each date reveals that December 24, 25, and 26, and March 2 and 3 are
high incidence periods. There are 2 chances in 10 that a severe winter storm will occur
somewhere in the state on these dates. In contrast, low storm incidence periods are
December 3-4, December 15-16, January 3-5, January 23-28 (normally the January thaw
period), February 20-24, March 15-17, and March 21-24. Although historically these are low
incidence periods, this does not mean that storms will never occur on these dates. In
fact, in 1977 two severe winter storms hit Illinois during the December 2-6 period.

A Typical Severe Winter Storm

Scientists at the Illinois State Water Survey studied 304
severe winter storms that occurred in the years 1900 to 1960. Some features of these
storms were averaged together to form a model of the typical severe winter storm in
Illinois. The model (Figure 3) depicts the pattern of freezing
rain, sleet, and snow that typically exists with severe winter storms. The snow area of 4
inches or more is about 215 miles long and 70 miles wide, lying with a southwest-to-northeast axis somewhere in Illinois.

Many severe winter storms strike Illinois will probably
not have the exact rain, sleet, and snow patterns depicted by this model. However, the
model is useful in that it shows us several relationships. First, the very heavy snowfall
(12 inches), if it occurs, usually is in a relatively small area that is less than 3
percent of the area affected by the storm.

The model also shows us that different types of
precipitation can fall in the same area. Note how the sleet area overlies the southern
half of the heavy snow area. The fall of snow is preceded sometimes by the sleet or occurs
at the same time as the sleet in the initial stages. Farther to the south is the freezing
rain zone, and the model shows it is not uncommon to have freezing rain and sleet
together. The center of the freezing rain zone is usually 50 miles or so south of the
6-inch snow area.

The Birth of an Illinois Storm

The storms that affect Illinois during the winter most
often do not develop in or near the state. Rather, these storms start to take shape
hundreds or even thousands of miles away. Low pressure systems traveling east from as far
away as Asia may move into the United States from off the Pacific Ocean. Many of these
disturbances end their travels in the mountainous Southwest, dying out as they cross the
rough terrain. Some, however, do make it over the mountains and redevelop just east of
the Rockies.

One of the most favored areas for this re-development is
in Colorado and the surrounding region. Water Survey climatologists discovered that this
area was where most of our severe winter storms in Illinois come from. There are also two
other areas that favor development of severe winter storms, although to a lesser extent.
These areas are the Province of Alberta (just east of the Canadian Rockies) and the Texas
Gulf Coast. As shown on the map in Figure 4, there are five
storm types for Illinois based on:

The source of the storm

The track the storm follows

Not all low pressure systems that develop in the
favorable areas during the winter become severe storms. Thankfully, only a relatively
small percentage of all lows that form, regenerate, or intensify in these three source
areas actually lead to severe winter storms in Illinois.

Storms tend to form where there are great contrasts in
temperature. During the winter months the lee side of the Rockies and the western Gulf
Coast provide these battlegrounds for the air masses. Once storms develop, the
winds in the upper atmosphere determine where and how fast they move, and ultimately if
Illinois will be their target.

The severe winter storms that affect Illinois tend to
follow certain paths. The most damaging storms, on the average, are those that originate
in Colorado and end up moving just south of Illinois (Types 2 and 4 on the map in Figure 4). Cold air north of the storm center in the Great Lakes or
upper Midwest can then feed into the storm - the necessary ingredient for snow or ice.

Precipitation with most major low pressure systems tends
to fall east and north of the storm center (see Figure 5). If
the air is cold enough in the winter, the precipitation will be snow, sleet, or freezing
rain. The reason the Type 2 and 4 storms usually result in the most damages is that the
paths they follow tend to keep Illinois in the area of maximum precipitation for the
greatest amount of time. In the record-breaking winter of 1978-1979, nine of the 18 severe
winter storms were Types 2 and 4, and five of the most damaging storms during the winter
were Types 2 and 4.

What About Snow?

When the word "snow" appears in a weather
forecast, it arouses a great deal of concern for most people, no matter what amount is
expected. One inch of snow may result in minor travel delays, while 10 inches can close
down a city.

One type of severe storm in Illinois produces a snowfall
of 6 inches or more in 48 hours or less somewhere in the state. However, the severity of a
particular snow storm is not measured solely by the amount of snow that falls, but also by:

The temperature at which it falls

Whether or not it is accompanied by high winds

The amount of water in a snowflake is determined largely
by the temperature of the air it forms in. If snow is falling with the air temperature at
32 °F, 1 inch of
that snow, if melted down, would yield on the average 0.10 inches of water. However, if the
snow was falling at, say, 10 °F, that 1 inch of snow would yield an average of only 0.05 inches of water
if melted. In other words, at lower temperatures a given amount of snow contains less
water, making it lighter and fluffier, than the same amount of snow at higher
temperatures.

Many of us know what it is like to shovel heavy, wet
snow from the sidewalk or driveway, and how comparatively easy it is to clear the same
amount of snow when it is much drier and lighter. This same relationship also makes it
easier for the wind to pick up snow from the ground. Snow rarely drifts when the
temperature is near the freezing point as the snow is falling. As the temperature falls
farther below freezing, however, the snow is prone to being blown into drifts.

Our typical severe storm: 6 inches or more of snowfall that occurs when the temperature is near freezing will result in considerable
inconvenience and disruption of daily activities, but the snow can normally be cleared from
roads and walks without too many problems. But a 6-inch snowfall at a temperature of 10 °F
with strong winds is
going to result in considerable blowing and drifting of snow, which will choke highways,
strand travelers, and isolate towns. Even after the snow stops falling, the snow on the
ground may continue to blow and drift for hours and perhaps days, depending on how long
the winds remain high. Conditions may be just as bad as when the snow was falling,
preventing snow removal from streets and highways.

The Worst of Winter - Freezing Rain

Freezing rain occurs when rain developing in a relatively
warm (above freezing) layer of air falls through a layer of air that is below freezing
(25-32 °F). The
rain is "supercooled" (still liquid) as it falls through the cold layer near the
surface of the earth. When the supercooled but still liquid raindrops strike the ground or
an object already below freezing, they freeze on contact. The resulting coating of ice is
commonly known as glaze.

A heavy accumulation of ice can topple power and
telephone lines, television towers, and trees. Highways become impossible to travel on,
and even stepping outdoors on foot can be an extremely risky undertaking. If you have
lived through an Illinois winter, chances are very good that you have already had some
encounter with freezing rain.

The severity of an ice storm (that is, the amount of
damage) depends on:

The amount of rain and thus icing taking place;

The strength of the wind;

Whether the storm strikes an urban or rural area.

Urban areas tend to suffer more damage than rural areas
because of the concentration of utilities and transportation systems (aircraft, trains,
buses, trucks, and cars), all of which may be affected to a great degree by the ice storm.

The area most likely to experience freezing rain in
Illinois is a west-southwest to east-northeast band extending from Pike County in western
Illinois to Iroquois and Vermillion Counties in eastern Illinois (see Figure 6).
This area corresponds well with the average motion of
ice storms. The worst icing situations tend to result from storm Types 2 and 5
(Figure 4), moving from the west-southwest or southwest. In a study
of 86 ice storms, the maximum number of storm motions were from the west-southwest.
Ice-only storms (no snow) tended to move from the southwest. On the average here in
Illinois, we will experience 15 severe ice storms in a 10-year period.

Recent Severe Winter Storms in Illinois

Since 1976, residents of Illinois have lived through
three of the most severe winters of the century. The winter of 1977-1978 holds the
distinction of being the worst, followed by the winter of 1981-1982 and then the winter of
1978-1979. These winters produced a total of 53 severe winter storms.

One of the most severe ice storms to hit central Illinois
since 1967 began the morning of March 24, 1978. Freezing rain continued until the morning
of March 25, and by the time the rain ended, 1/2 to 2 inches of ice
coated a 90-mile-wide belt west to east across central Illinois. One million people were
without power at least 24 hours, and some outages took up to two weeks to repair. Over
1000 auto accidents occurred, and there was an estimated $20 million in tree losses.
Twenty-four counties in Illinois were declared disaster areas. Figure
7 shows the snow and ice pattern for this storm.

The winter of 1981-1982, the second most severe winter in
this century, featured 18 severe winter storms, 8 in January alone. The most damaging
storm of that winter struck on January 29-31, 1982. Ten to twenty inches of snow fell on
an area extending from the southwestern counties of the state to east-central Illinois
(Figure 7). The storm resulted in 10 deaths, and the National Guard
was called out to help with snow removal in southern Illinois. In the ten days following
this storm, these same areas received another 5 to 15 inches of snow, leading the
Governor of Illinois to declare 15 southern Illinois counties disaster areas.

The Impacts of Winter Storms

The disabling effects of large amounts of snow or ice on
daily activities are quite extensive. Metropolitan transportation systems are usually
affected first and hardest. Transportation is the backbone of our economy, and the storm
effects lead to impacts on domestic and commercial activities.

Losses due to a major winter storm may result from lost
revenue to the state from people not working and businesses closed (lost taxes) and from
increased expenses for snow removal and salting operations. The delivery of goods and
services may be sharply curtailed. The effects of a severe winter storm may impact many
facets of our lives.

However, not all the impacts are detrimental. A bad
winter storm keeps people indoors. Crime rates tend to drop (although police are kept busy
with accident calls), and the incidence of colds and flu may sometimes be reduced because
people have to stay indoors and public meetings or other gatherings are called off. Stores
may profit from sales of sleds, toboggans, snowmobiles, and skis. Sales of snow shovels,
snow blowers, and snow tires typically increase, especially if the storm occurs early in
the winter. However, the undesirable impacts of these storms far outweigh the desirable
ones.

How A Severe Winter Storm Can Impact Our Lives

Highways closed or difficult to travel on

Airports closed, business and pleasure trips cancelled or delayed

Commuter trains and buses delayed

Mail delayed

Difficulty in obtaining medical care (Unable to get to
hospital - ambulances blocked by snow-covered roads)

Shipment of food and other goods delayed

Schools closed

Businesses closed

Stores lose business because patrons unable to get there

Power and communications disrupted because of downed lines

Fire and police delayed in responding to emergencies

Cars hard to start, need assistance (often extra costs)

Increased auto accidents

Large costs to state, county, and local highway
departments for snow removal

References

Much of the material in this report was based on research
at the Illinois State Water Survey and is described in more detail in the following Water
Survey publications: